The present invention relates to a complex oxide having an excellent thermoelectric conversion performance.
In our country, effective energy is obtained at a ratio of only about 30% from the primary supply energy, and about 70% of energy is eventually discarded as heat into the atmosphere. The heat evolved by combustion in plants, garbage-incinerating facilities or the like is thrown away into the atmosphere without conversion into other energy. In this way, we are uselessly casting away a vast amount of thermal energy and are acquiring only a small amount of energy by combustion of fossil fuel or otherwise.
To increase the proportion of acquired energy, the thermal energy to be released into the atmosphere should be effectively utilized. For this purpose, thermoelectric conversion for direct conversion of thermal energy to electrical energy is effective means. The thermoelectric conversion, which utilizes Seebeck effect, is an energy conversion method for generating electricity by creating a difference in temperature between both ends of a thermoelectric material to produce a difference of electric potential. In this thermoelectric generation, electricity is generated simply by setting one end of a thermoelectric material at a location heated to a high temperature by waste heat, and the other end thereof in the atmosphere (room temperature) and connecting a leading wire to both ends thereof. This method entirely eliminates a need for a device with moving parts such as a motor or a turbine required in the common generation of power. As a consequence, the method is economical and can be carried out without giving off a gas by combustion. Moreover, the method can continuously generate electricity until the thermoelectric material is deteriorated.
As set forth above, thermoelectric generation is a technique expected to play a share of role for the resolution of energy problems which will be concerned henceforth. To realize the thermoelectric generation, there is a need for developing thermoelectric materials which have a high thermoelectric conversion efficiency and have excellent heat resistance and chemical durability. Currently, intermetallic compounds are known as a substance of high thermoelectric conversion efficiency. Among them, Bi2Te3 has the highest thermoelectric conversion efficiency. However, the thermoelectric conversion efficiency of Bi2Te3 is only about 10% at highest. Further, Bi2Te3 can be used only at a temperature of 200xc2x0 C. or lower. TeAgSb-containing metallic compound has a high thermoelectric conversion efficiency in a temperature range of about 600 to about 1,000 K which falls in the temperature range of waste heat. However, Te and Sb are rare elements having toxicity and can not be used in the air because they are readily oxidizable. With the above drawbacks, Bi2Te3 and TeAgSb-containing metallic compound are limited in application as a thermoelectric material for use.
In the foregoing situation, it is expected to develop materials composed of low-toxicity elements which exist in large amounts and which have superior heat resistance and chemical durability and a high thermoelectric conversion efficiency.
While metallic oxides may be proposed as materials excellent in heat resistance and chemical durability, the metallic oxides are lower in thermoelectric conversion efficiency by an order of magnitude than Bi2Te3. In fact, known oxides having a high electric conductivity (i.e., an electrical resistivity of about 10 mxcexa9cm or less) show a Seebeck coefficient only as low as tens xcexcV/K or less.